An air pressure adjusting device adjusts the air pressure of pneumatic tires mounted on a vehicle. While the vehicle is traveling, the air pressure adjusting device adjusts the air pressure of the pneumatic tires based on a target air pressure calculated according to the travel conditions of the vehicle (for example, vehicle velocity, travel route, road surface conditions, and the like). By so doing, running performance and fuel consumption of the vehicle are improved. A known conventional air pressure adjusting device of this type is the art described in Japanese Unexamined Patent Application Publication No. 2009-056948. A conventional air pressure adjusting device has an air passage in the wheel for introducing compressed air into a pneumatic tire.